In erasable ball-point pen ink as described in, for example, U.S. Pat. No. 4,097,290, 4,329,264, 4,329,262, it is generally required that
(1) in order to render ink traces erasable, a pigment, not a dye, must be used as a coloring agent for ink, and in addition a rubber must be used and, at the same time, ink per se must be provided with an extremely high viscosity, e.g., as high as 1,000,000 to 6,000,000 cps.
On the other hand, in ball-point pens to make it possible to write fine letters, it is generally required that
(2) in order to make it possible to write fine letters, ball-point pens must be provided with a writing ball having a small diameter (.e.g., not more than 0.8 mm) and a narrow clearance (ca. 3 microns) between the writing ball and the ball holder.
However, when both of the above requirements (1) and (2) are satisfied in a ball-point pen at the same time, the following disadvantages in writing performance are known to exist.
(3) Satisfactory writing properties could hardly be obtained since ink traces become faint, thin or discontinued;
(4) When used over a long period of time (say more than a writing length of 400 to 1000 m), the ball holder of the ball-point pen may be worn to such an extent that distinctly uneven ink traces are produced, globs of ink due to overflow thereof are formed and, in extreme cases, the writing ball falls out of the ball holder, making the pen completely useless;
(5) In cases where an ink having a relatively low viscosity and a fairly large stringiness is used, so-called "string-forming phenomenon" is liable to occur, thereby forming strings of the ink from the writing tip like a spider's thread; and
(6) When a ball-point pen remained unused for some time, the surface of writing ball of the ball-point pen is covered with a film of dried ink, thus impairing initial writing properties when the ball-point pen is reused.